A motor drive apparatus disclosed in the following patent document 1 has a plurality of inverters. In this motor drive apparatus, when one of the plurality of inverters fails, electric power is stopped from being supplied from a failing inverter to a plurality of coil sets and electric power is supplied to the coil sets from normal inverters other than the failing inverter. Thus, even when one of the inverters fails, the motor can be continuously operated by driving the motor by only the normal inverters.    [Patent document 1] JP 2005-304119A
In a motor drive apparatus, a combination of one inverter and one coil set, which is paired to such an inverter, forms one power supply system. According to a prior art technology disclosed in patent document 1, when power supply to a failing system is stopped in case of detection of failure, output of the failing system falls at the same time as the occurrence of failure. As a result, motor operation rapidly changes immediately after the failure. In case that the motor drive apparatus is used, for example, as an electric power steering system for a vehicle, the rapid change in motor output will cause vehicle operation change, which a driver do not intend. It is possible to continuously compensate for the loss in motor output by the normal systems other than the failing system. The inverters in the normal systems will be loaded excessively and overheats. Although it is also possible to increase power capacity of inverters to protect the inverters from being overloaded, the inverters become large-sized and expensive.
The applicant of the present application proposed technologies, which counter the foregoing problem by two patent applications. One is a U.S. patent application Ser. No. 12/977,449 (JP application No. 2009-295533). This technology provides a motor drive apparatus, which suppresses operation changes caused by stopping an operation of a failing system upon occurrence of a failure in one of power supply systems. This is a solution to solve the problem that a rapid operation change occurs immediately after failure.
In case this technology is used in an electric power steering system of a vehicle, the operation change is suppressed by controlling inverters operating normally to compensate for loss of power, which has been supplied by a failing system, temporarily at the time of occurrence of failure. Since a driver will not sense any change in steering operation in such a case, the driver will not notice the failure.
Under a condition that one of two power supply systems of motor drive apparatus is in failure, the driver notices the failure and generally takes the vehicle to a repair shop at the earliest time. This technology intends to continue to drive the motor by one normal system so that the driver will be able to drive the vehicle to the repair shop with the same steering feeling as before the occurrence of failure. If the driver continues to use the vehicle without noticing the failure, the normal system will also fail sooner or later and steering torque will not be power-assisted at all in the end.
The second one is a U.S. patent application Ser. No. 12/977,489 (JP application No. 2009-295534). This technology provides an electric power steering system, which equalizes a maximum current command value commanded to an inverter (power converter) of a normal system after occurrence of failure to a maximum current supply value having been supplied to coils by the inverter of the normal system before occurrence of failure. This is a solution to solve the problem that the inverter of the normal system will be overloaded and overheat if it is controlled to maintain even after the occurrence of failure the same total output as before the occurrence of failure.
For example, in case that one of two systems fails, the maximum current command value is reduced to one-half from that of two systems. Thus, the inverter of the normal system is protected from being overloaded. Further, a change in the feeling of steering operation is provided to the driver by reducing the steering torque output to one-half. It is also proposed as an auxiliary measure to call driver's attention by a warning light or a buzzer at the time of failure.
The output required in the electric power steering system is reduced, however, to about one-third of output required when the vehicle is at rest, when travel speed of the vehicle becomes higher than 4 km/hr. For this reason, even if the steering assist torque output is reduced to one-half during travel of the vehicle, the driver will not sense change in steering operation feeling until a steering wheel is turned a large angle. The driver will thus not notice the occurrence of failure.
The following problem will also arise when the vehicle starts to travel from the travel stop condition in a parking lot or at a traffic light, if the steering assist torque is reduced to one-half. It is because high power output is required most at the time of starting to travel.
First, the steering assist torque provided is insufficient. The motor for the electric power steering system is generally designed to have specifications, which meet a maximum torque requirement. That is, in case that the motor drive apparatus has two power supply systems, the motor is designed to generate the maximum required torque by two power supply systems. Therefore, the output by only one power supply system cannot provide sufficient steering torque.
Since sufficient steering assist torque cannot be provided, a driver is required to perform heavy steering operation and hence need long time to steer a vehicle. The time, in which a current continues to flow to drive the motor, becomes long. For this reason, even if the output is reduced to one-half relative to that of normal time, the amount of electric power increases to be more than twice when the current supply time becomes more than twice. The inverter in the normal system will tend to overheat. It is thus impossible to reduce abnormal heat generation of the inverter operating normally.
In case that the motor drive apparatus has an overheat protection control function, which detects temperature of the inverter or the like and limits a current command value when the detected temperature rises above a predetermined assured temperature value, the inverter of the normal system overheats by steering operation of the driver by force at the travel stop time of the vehicle. The current command value is limited by the overheat protection control function. As a result, the steering assist torque is reduced to be less than one-half of that of the normal time even after the vehicle started traveling. It is thus not possible that a driver can drive a vehicle with the same steering feeling as before the occurrence of failure with the steering assist torque, which is generated by continuing to drive the motor with one-half of power of the normal time while preventing excessive heating of the inverter of the normal system.